JSFiddle - React, Tailwind, and code Playground

HTML

<a href="http://google.com">Google</a> <br />
<a href="http://yahoo.com" onclick="alert('Yahoo!!')">Yahoo</a> <br />

JavaScript

function onclickHandler() {
    alert(this.href);
    return false; //so we can keep testing
}

window.onload = function() {
    var aTags = document.getElementsByTagName('A');
    for (var i = aTags.length - 1; i >= 0; i--) {
        if (aTags[i].onclick) {
            var oldOnClick = aTags[i].onclick;
            aTags[i].onclick = function() {
                onclickHandler.call(this);
                oldOnClick.call(this);
            }
        } else {
            aTags[i].onclick = onclickHandler;
        }
    }
}